One of the major sources of problems for the oil and coal processing industry and for coal, coke and oil uses is the presence of metals and sulfur. These contaminants poison the catalysts normally utilized during refining processes, mainly for cracking of heavy hydrocarbons present in crude oils. Also the presence of metals and sulfur in fuel oils, coal or coke produces serious environmental pollution following combustion.
Vanadium is preferentially found in crude oil or in coal originated in South America. In the United States the largest concentration of vanadium in the atmosphere occurs where residual fuels of high vanadium content from Venezuela are burned in utility boilers. Also coal ash in the atmosphere, originating from the burning of coke-like materials, contains vanadium.
There are two main reasons to promote the development of metal and sulfur recovery from oil, coal and coke materials. One is the present day concern over the quality of the air, and the second is the necessity to improve new processing methods to face increasing complexity on the chemical composition and structure of the remained deposits.
Also the high level contents of V, Co, or Ni, which often are present in crude oil, coal, coke or their derivatives encourages their recovery from an economic standpoint, especially in view of the actual high prices these metals show in the market.
However, an air pollution-free process for recovering metals from crude oils, coal, coke and related materials, which is also economically feasible with the present day refining methods has not materialized. The problem which has plagued industry is the capital cost associated with the equipment and the method designed to remove such contaminants.
Several methods have been proposed for removing metals (demetallation) and sulfur (desulfurization) from heavy oils and coals. Both metals and sulfur represent an environmental hazard in addition to the difficulties they produce during catalytic processing of crude oil. As an example, the light crude oil deposits in Venezuela are being rapidly depleted and today almost all oil deposits are of heavy and ultra-heavy nature.
The most important metals present in petroleum are nickel and vanadium. V concentration may vary from a small quantity such as 0.01 ppm to large amounts such as 10,000 ppm, and generally is more abundant than Ni, with the exception of crude oil from Africa or Indonesia.
Ni and V are found in crude oil forming two types of metallic compounds: Porphyrin complexes and non-porphyrin complexes. Porphyrin metallic complexes are the most difficult to remove and have been extensively studied because they distill at high boiling point, and also due to their attractive geo-chemistry.
Much research has been done to eliminate metal and sulfur from oil. A number of mineral acids have been used for demetallation purposes. Exxon Company showed that liquid hydrofluoric acid (HF) is an effective demetallizing agent, by extracting V and Ni as an insoluble precipitate. However, HF modifies substantially the chemical structure of the organic matrix in the oil.
Other chemical agents such as chlorine (Cl.sub.2), sulfuryl chlorine (SO.sub.2 Cl.sub.2), nitrogen dioxide (N.sub.2 O.sub.4), hydroperoxide, and benzoyl peroxide have been also tested. However, direct use of such strong oxidants diminishes the quality of the oil since they modify the chemical structure or composition of organic molecules. Even though Cl.sub.2 has proven to be one of the most efficient demetallizating agents, when directly used it produces undesirable addition reactions with some organic molecules. In general, it has been pointed out that oxidants like peracetic acid, sodium hypochlorite and chlorine readily attack the metal-porphyrin complex and extract the metal, but their use has not been successfully accomplished.
Metals are strongly chelated or complexed with organic ligands, preferentially porphyrins (metallo-porphyrins) and heterocyclic molecules containing S, N and O. Their removal is important and constitutes a key factor determining the success or lack of success of a given industrial oil refining operation.
Porphyrins present in petroleum originated in ancient chlorophyll. Through aging, V and Ni exclude Mg from its chlorophyllic frame taking its place. This can be represented by the FIG. 1, adapted from T. F. Yen. ("Trace Substances in Environmental Health", Vol. IV, D. D. Hemphil, Ed., Columbia University of Missouri Press, 1973). It is shown how the chlorophyll is gradually transformed to deoxofiloeritrine an active molecule for chelating V forming a DPED compound which contains chelated V.
At its turn DPED reaches an equilibria with a number of V containing porphyrins as it is partially shown in FIG. 1.
Experts say that coal is a major source of energy and will continue to be so for many years. However, coal contains sulfur, nitrogen and others impurities such as mercury, beryllium and arsenic. These constitute a health hazard and, therefore, coal must be cleaned either before, during, or after combustion to prevent deterioration of environment.
One of the major contaminants which has received deep attention is sulfur. Many desulfurization processes have been developed. Sulfur is present in coal in amounts ranging from traces to 10% as sulfate, pyritic and organic sulfur. The U.S. governmental regulations of atmospheric emission of sulfur oxides from coal combustion have focused on sulfur content reduction.
Physical cleaning and chemical cleaning is currently practiced throughout the coal industry. Chemical cleaning processes which remove a major portion of the sulfur are in the early stages of development and are not yet practiced commercially due to costs.
However, since the world must turn to coal as its major source of energy (the reserves of gas and petroleum are dwindling and expected to be depleted within the next 40-60 years) new, efficient and non-polluting methods need to be developed. Physical separation of sulfur is inadequate; only a portion of the pyritic sulfur and none of the organic sulfur can be removed without high coal losses. On the contrary, chemical cleaning methods available so far can achieve essentially complete removal of the sulfate and pyritic sulfur and up to 50% of the organic sulfur.
Several processes at present can achieve that degree of cleaning. Among them it can be mentioned: ferric-salt leaching, nitrogen dioxide oxidative cleaning, oxidative desulfurization, hydrogen peroxide-sulfuric acid leaching, hydrodesulfurization, etc. Most of these and other chemical cleaning processes are still in the early stages of development.
The method herein disclosed to recover metals and to eliminate sulfur is based on the oxidating effect of hypochlorous acid which is released in situ upon combining a hypochlorite salt solution with a mineral strong acid. The chemical reactions operating between this acid mixture and the metals and sulfur present in the material produce a high demetallation and desulfurization yield, but without affecting the structure of the organic matrix in the case of oil materials. The method can be conveniently adapted to the cleaning of coal, especially to those which possess valuable metals susceptible to being recovered.